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Abstract

The basic purpose of a file access processor is to manage a set of objects (files within directories) for a set of users. These objects may be shared, requiring a high degree of concurrency for a file access processor. Because the environment allows sharing, a mechanism must be established that will ensure concurrency during sharing. This concurrency is based on a compatibility scheme devised for System R and Structured Query Language/Data System (SQL/DS) with the following extension: For each file define a logical object identifier. Hold locks on catalog (relational) rows only while obtaining the lock on the logical object and during commit.

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United States

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English (United States)

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This is the abbreviated version, containing approximately
57% of the total text.

The basic purpose of a file access processor is to manage
a set of objects (files within directories) for a set of users.

These objects may be shared, requiring a high degree of concurrency
for a file access processor. Because the environment allows sharing,
a mechanism must be established that will ensure concurrency during
sharing. This concurrency is based on a compatibility scheme devised
for System R and Structured Query Language/Data System (SQL/DS) with
the following extension:
For each file define a logical object identifier. Hold

locks on catalog (relational) rows only while obtaining

the lock on the logical object and during commit.

The legend of the lock matrices is as follows:

Modes:IS = Intent Share

IX = Intent Exclusive

S = Share

X = Exclusive

CS = Checkout Share

CU = Checkout Update

CX = Checkout Exclusive

The length of time (duration) that a lock can be held is one of the following
possibilities:

(Image Omitted)

INSTANT - The lock is acquired and then immediately freed. SHORT The lock
is held for the duration of the storage access control (SAC) operation, e.g., SAC
FETCH, SAC NEXT. MEDIUM: The lock is held over multiple SAC operations,
but may be freed before the end of a logical unit of work (LUW). LONG: The lock
is held until the end of the LUW.

The locking protocol defined in this document and the interaction with
Checkout Locking is based on the compatibility matrix [*, p. 393] shown in Fig. 1.
Note that all long locks are kept until end-of-LUW (commit or rollback). All
medium locks are released during op, at close (for read) or end-of-LUW.

In Fig. 1, if holder and requester are the same, then the request is granted
except in the case of conflicting explicit (checkout) locks.

A set of rules were established that determined how the different File
Processor requests were to interact, and how the processor was to be perceived
externally. The rules are:

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1

Page 2 of 6

1. Readers never wait. Whenever possible, let a reader's request be
handled immediately. A writer may have to wait occasionally if the request
conflicts with another request already in progress.

2. The object ID associated with each object in the File Processor must be
unique. In order to maximize concurrency without interfering unnecessarily with
other requests, each object operated on must be uniquely identified from other
objects in the system.

(Image Omitted)

3. Hold relational locks until all logical locks are obtained. Minimize the length
of time relational locks are held as much as possible. In order to get the current
information about the object referred to in the current request, we must 'read' a
catalog row. The process of reading the row will get a relational lock. This lock
prevents updates from occurring until it is released. More information is provided
on page 5 where a method t...